I've seen the pensar vs achar topic elsewhere, but I cannot decide which is best in this circumstance.
You can use them interchangeably ;)
If you know french it is more understandable: achar = trouver (to find), so when you need to use find as a synonym to think, use achar:
Ex: I find that woman pretty = Eu acho que essa mulher é bonita
I think that we can go = Eu penso que podemos ir
Tue second one seems more natural if you use "achar"
In spanish is even more intuitive. Achar = Hallar, the use is identical and the pronunciation is extremely close.
Is "Ela vai pensar que Eu não amo ela" wrong here?
It sounds too bad to my ears.
What about "nao amo a ela"?
"amar" does not require a preposition. It should be
Is there a general rule for when to use "o que" and when to use "que" by itself?
O que = what. Que = that, what a/an.
Thanks. My translation for "She is going to think that I do not love her." was "Ela vai pensar o que eu não a amo." and it was marked as incorrect. Are "o que" and "que" interchangeable in this case?
No, they are not interchangeable most of the time. And here you use "que" =)
I still have to pay a lot more attention to distinguish "ele" from "ela." So much easier in Spanish. :(
When should I use '...a amo' vs '...a amo-lhe'? Also, how do you differentiate sex when using 'lhe'?
Eu o amo = I love him/it. Eu a amo = I love her/it. There is not "lhe amo" / "amo-lhe". You know by the context if lhe refers to a man or woman. (Lhe is used for indiret verbs).
Ela vai pensar que eu não se amo. Why a amo ?